1. Field of the Invention
The present invention is related to an optical modulating apparatus for intensity/phase-modulating an optical signal in a batch mode by driving a phase-modulating electric signal and an intensity-modulating electric signal.
2. Description of the Related Art
In conventional optical modulating apparatuses, the direct modulating system has been utilized in which a semiconductor laser is modulated by the drive current to produce an optical intensity signal directly proportional to the electric signal. However, in long-distance/wide-range optical transmission systems capable of transmitting optical signals over several thousands Km at transmission speed of several Gbp, or higher, optical modulation waveforms are largely deformed due to the chirping phenomenon, and the wavelength dispersion and the non-linear characteristic of the transmission path, so that the resulting transmission capacity is limited. In this chirping phenomenon, the wavelength of light is changed during the direct modulation.
On the other hand, since the chirping phenomenon is very little and the operation range higher than 10 GHz can be relatively simply achieved in the external modulating system, this external modulating system starts to be applied to large-capacity optical communication systems. Furthermore, in order to minimize deterioration in light waveforms after light signals have been transferred over long distances, optical phase modulation is intentionally superimposed on optical intensity modulation.
As the typical external modulator, a lithiumNiobate (LiNbO.sub.3)-MachZehnder type optical modulator (will be referred to as an "LN-MZ type optical modulator" hereinafter) is known which is a type of MachZehnder type optical modulator.
An LN-MZ type optical modulator is operated as follows. An input optical signal is split into two optical signals inside this LN-MZ type optical modulator. While a phase modulation region is provided in both or either one of the two split optical signals, a phase of light to be transmitted is changed, and thereafter the phase-changed optical signals are again synthesized with each other. Then, the synthesized optical signal is outputted.
A phase adjustment is performed by way of a phase modulation by the electrooptic effect. Depending upon the phase change in the light in the phase modulation region, the interference conditions of the light are changed when the light signals are synthesized with each other at the post stage, and both the intensity and the phase of the light can be modulated. In such a case where phase modulations are provided in both of the two branch paths (two electrodes), both the intensity and the phase of the output light can arbitrarily be changed.
Now, a description will be made of operations of an LN-MZ type optical modulator having two electrodes. Assuming now that an electric field of input light is Ei(t); a field strength is A; modulation signals inputted into electrodes of a phase adjustment region are S1(t) and S2(t); DC bias voltages applied to electrodes in the respective branch paths are Vb1 and Vb2; and an angular velocity of light is ".omega.", an electric field Eo(t) after the light signals are combined with each other is expressed by the following formula (1): ##EQU1##
In this formula (1), symbol ".alpha." shows a fixed phase change amount in the phase modulation region; and symbol ".beta." represents a sensitivity of a phase modulation with respect to the input signal. It is assumed that ".alpha." and ".beta." in each of the 2-split phase adjustment regions are equal to each other. Also, symbol "exp(i.alpha.)" shows a fixed phase delay of a modulator, and is a constant.
Now, when the modulation signals S1 and S2 are defined by the following formula (2), the electric field Eo(t) may be summarized by the following formula (3): EQU S1(t)=P(t)+A(t), EQU S2(t)=P(t)-A(t) (2) EQU Eo(t)=exp {i.multidot..beta..multidot.(P(t)+(Vb1+Vb2)/2)}.multidot.cos {.beta..multidot.(A(t)+(Vb1-Vb2)/2)}.multidot.Ei(t).multidot.exp {i.alpha.)(3)
A first item of a right hand in the above-described formula (3) indicates an item for representing a phase modulating component by P(t), and a second item of the right hand thereof is an item indicative of an intensity modulation component by A(t). It can be seen that the phase modulating operation and the intensity modulating operation are independently carried out by P(t) and A(t).
As previously explained, if the 2-electrode LN-MZ type optical modulator is used, both the phase modulating operation and the intensity modulating operation can be simultaneously achieved. To achieve an arbitrary phase modulation and an arbitrary intensity modulation, such a signal converter is required which may convert the phase modulation signal P(t) and the intensity modulation signal A(t) into an electric signal S1(t) and an electric signal S2(t), which are applied to an optical modulator.
As such a signal converting means, for instance, one signal converting means is described in IEEE Journal of Selected Topics in Quantum Electronics, Vol.2, No.2, JUNE 1996, P300-P310, "Pulse Generation for Soliton Systems Using Lithium Niobate Modulators"
FIG. 3 is a schematic block diagram in which the structural diagram of the signal converting means described in the above-described reference document is modified. In FIG. 3, reference numeral 1 shows a 2-electrode LN-MZ type optical modulator, reference numerals 2 and 3 indicate 3-dB splitters for splitting an electric signal into two electric signals, and reference numerals 4 and 5 show phase shifters for adjusting a phase of an electric signal. Reference numerals 6 and 7 indicate combiners for combining two electric signals with each other, reference numerals 8 and 9 are terminating devices for terminating an electric signal so as to prevent this electric signal from being reflected, and reference numeral 15 shows a light source for outputting light having constant intensity to be entered into the optical modulator 1. For the sake of simple explanations, it is assumed that a delayed length of each of connection lines is 0, and a loss of each of blocks is 0.
Next, operation will now be explained. This signal converting means of the above-described reference document is directed to such a signal converting operation that both the phase modulating operation directly proportional to P'(t) and the intensity modulating operation directly proportional to cos{K A'(t)}(symbol K being constant) are simultaneously performed by using the phase modulation signal P'(t) and the intensity modulation signal A'(t).
The voltage amplitude of the intensity modulation signal A'(t) inputted into the 3-dB splitter 2 is attenuated to 2.sup.-1/2, and the attenuated intensity modulation signal A'(t) is split into two sets of intensity modulation signals having the in-phases with respect to that of the inputted intensity modulation signal to become 2.sup.-1/2 A'(t). In one path, this intensity modulation signal is delayed by the phase of ".pi." by the phase shifter 4, so that a signal of -2.sup.-1/2 .multidot.A'(t) is inputted into the combiner 6. In the other path, such a signal of 2.sup.-1/2 .multidot.A'(t) is inputted into the combiner 7.
On the other hand, similarly, the voltage amplitude of the phase modulation signal P'(t) inputted into the 3-dB splitter 3 is attenuated to 2.sup.-1/2, and the attenuated phase modulation signal P'(t) is split into two sets of intensity modulation signals having the in-phases with respect to that of the inputted intensity modulation signal to become 2.sup.-1/2 .multidot.P'(t). In one path, this phase modulation signal of 2.sup.-1/2 .multidot.P'(t) is directly inputted into the combiner 6. In the phase shifter 5 of the other path, the phase fluctuation is adjusted (namely, phase change is set to 0), so that such a signal of -2.sup.-1/2 .multidot.P'(t) is inputted into the combiner 7.
Since the voltage values of two sets of the input signals whose amplitudes have been attenuated to 2.sup.-1/2 and thereafter added to each other are outputted from the combiners 6 and 7, the output signal S1(t) from the combiner 6 and the output signal S2(t) from the combiner 7 are expressed by the following formulae (4) and (5): EQU S1(t)=1/2.multidot.A'(t)+1/2.multidot.P'(t) (4), EQU S2(t)=-1/2.multidot.A'(t)+1/2.multidot.P'(t) (5).
Paying attention to such a conversion from A'(t) and P'(t) into S1(t) and S2(t) in the above-described formulae, it may be seen that the amplitude of the voltage inputted into the splitter is attenuated to 1/2(=-6 dB) at the output point from the combiner.
The above-described prior art discloses such a technique that the electric signals S1(t) and S2(t) which are applied to the two electrodes of the optical modulator 1 are produced from the phase modulation signal P'(t) and the intensity modulation signal A'(t) by employing the 3-dB splitter, the phase shifter, and the combiner when both the phase modulating operation and the intensity modulating operation are simultaneously carried out by the 2-electrode MZ-LN type optical modulator.
However, since a loss occurred in the 3-dB splitters and the combiners become -3 dB (total amount of loss is -6 db, namely, a half of voltage), an amplifier having a large output gain is required. On the other hand, as to a high output power/wide-band amplifier operable in high-speed optical communications at frequencies higher than several GHz, as the output power of this amplifier is increased, there arise various problems in view of heat dissipation, cost, and reliability of this high output power/wide-band amplifier. As a result, it is very important to reduce the output level of such an amplifier.
Furthermore, since the total numbers of components constituting the 3-dB splitters and the combiners are very large, the packaging scale thereof is increased and therefore the adjustment is required in the connection length of these components. In the case where as a phase shifter for adjusting a phase of a high frequency electric signal, a variable delay line is used in which a length of a coaxial line is varied, an amount of phase change caused by changing the same coaxial length becomes different, depending upon a frequency. As a consequence, there is such a drawback that the phase shift amount of the phase shifter must be readjusted with respect to each of the operation frequencies when the sinusoidal wave modulation is carried out.
Also, assuming now that the intensity modulating operation is performed by such a wave having a plurality of frequency components other than a sinusoidal wave, since the phase shift of ".pi." must be applied to all of the frequency components, the phase shifter cannot be constituted by employing the variable delay line. Therefore, such a phase shifting means as a 180-degree hybrid circuit is required by which a 180-degree phase shift can be made as to all of frequencies within a signal band.